<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  
  <script type="text/javascript">
    /*
      then参数中的函数返回值
    */
    function queryData(url) {
      return new Promise(function(resolve, reject){
        var xhr = new XMLHttpRequest();
        xhr.onreadystatechange = function(){
          if(xhr.readyState != 4) return;
          if(xhr.readyState == 4 && xhr.status == 200) {
            // 处理正常的情况
            resolve(xhr.responseText);
          }else{
            // 处理异常情况
            reject('服务器错误');
          }
        };
        xhr.open('get', url);
        xhr.send(null);
      });
    }
    queryData('http://localhost:3000/data')
      .then(function(data){
        return queryData('http://localhost:3000/data1');
      })
      .then(function(data){
        return new Promise(function(resolve, reject){
          setTimeout(function(){
            resolve(123);
          },1000)
        });
      })
      .then(function(data){
        return 'hello';
      })
      .then(function(data){
        console.log(data)
      })

  </script>
</body>
</html>